1. Field of the Invention
The present invention relates to a Schmitt trigger circuit in which a malfunction due to variance in input signal voltage and internal voltage can be prevented.
2. Description of the Related Art
In general, a Schmitt trigger circuit is used as an interface between an analog circuit and a digital circuit. In other words, the Schmitt trigger circuit utilizes its inherent hysteresis characteristic, and converts an analog voltage waveform which varies gradually into a digital waveform which varies sharply, thus eliminating noise and parasitic oscillation components of the analog voltage. For performing this conversion, the Schmitt trigger circuit is designed so as to have a hysteresis characteristic between high and low threshold voltage levels. In designing the Schmitt trigger circuit, it is necessary that the high and low threshold voltages and a hysteresis voltage be set to desired values and these values have good repeatability.
FIG. 7 shows an example of a conventional Schmitt trigger circuit. In FIG. 7, DC voltage source 71 provides bias to input signal Vin. Operational amplifier 72 compares input signal Vin with high-level constant threshold voltage VA. Operational amplifier 73 compares input signal Vin with low-level constant threshold voltage VC. Voltage divider 74 comprises a plurality of resistors, which are series-connected between power source voltage Vcc and ground, and produces two-level threshold voltages VA and VC. Set/reset (RS) type flip-flop circuit 75 receives an output of operational amplifier 72 at set input S and receives an output of operational amplifier 73 at reset input R.
In the above-described circuit, if input signal Vin is supplied in accordance with the timing chart of FIG. 8. When the voltage of input signal Vin becomes higher than high-level side threshold voltage VA, operational amplifier 72 produces a "1" level signal. When the voltage of input signal Vin becomes lower than low-level side threshold voltage VC, the other operational amplifier 73 produces a "1" level signal. In other cases, the operational amplifiers produce "0" level signals. As a result, set signal S and reset signal R supplied to flip-flop circuit 75 vary, as shown in FIG. 8. By these set and reset signals, flip-flop circuit 75 is set and reset. Input signal Vin is thus converted to digital signal Sout through flip-flop circuit 75.
Now consider the case where a reproduced VTR control pulse signal is supplied, as input signal Vin, to the Schmitt trigger circuit. The level of the reproduced VTR control pulse signal changes according to variance in a record current for effecting recording on a magnetic tape, the speed of running of the tape, and characteristics of the tape. Mechanical noise, parasitic oscillation components or rippled components of power source voltages, and variant DC components in the electric circuit are superposed on the reproduced pulse signal. Considering these superposed components, two levels of threshold voltage VA and VC must be provided in the conventional art. However, in the event that the level of the reproduced control pulse signal and the level o the noise signal are both high while the levels of threshold voltages VA and VC are both low, as shown in FIG. 9, set signal S and reset signal R are erroneously produced from operational amplifiers 72 and 73 in periods in which pulses are, in fact, not supplied. As a result, converted digital signal Sout has an erroneous waveform. On the other hand, in the event that the level of the reproduced control pulse signal and the level of the noise signal are both low and do not reach the threshold levels, as shown in FIG. 10, set signal S and reset signal R are not produced, resulting in an erroneous waveform of digital output signal Sout.
In the circuit structure of FIG. 7, suitable two threshold voltage levels must be set in order to prevent an erroneous digital output from being produced. However, this setting of the suitable levels is very difficult, since various conditions must be considered.
As described above, it is difficult to set suitable threshold voltage levels in the Schmitt trigger circuit of FIG. 7. In this circuit, variance in the internal power source voltage may lead to malfunction.